1. Field of the Invention
The present invention relates to a hybrid combustion burner using catalytic combustion appropriate to combustion conditions of high temperature and high pressure. More particularly, the invention relates to a hybrid high pressure combustion burner employing catalysts and CST (Catalytically Stabilized Thermal, hereafter called as xe2x80x9cCSTxe2x80x9d) combustion with staged mixing systems, in which a primary combustion gas containing reacted gas of a primary gas mixture, which is formed via catalytic combustion, is subject to a secondary mixing with the primary gas mixture containing fuel and high temperature air, completing stable high temperature CST combustion at ultra lean conditions, thereby achieving high temperature combustion reducing the formation of NOx.
2. Description of the Related Art
The ratio of CO2 amounts discharged by combustion of coal, petroleum and natural gas is considered to be 10:8:6. Natural gas is the cleanest fuel and generates the least CO2, among available fossil fuels. Accordingly, Natural Gas Research in the U.S. anticipates that about 50% of the world""s generated energy will be provided by gas turbines within the next 20 years. As anticipated, demands for gas turbines are increasing with time.
Technologies for reducing and removing NOx can be grouped into combustion modifications and post-combustion processes. Especially, as for the post-combustion processes, SCR (Selective Catalytic Reduction) is the most commonly used. However, this has a problem of very high operation costs, due to requirement of ammonia employed as a reducing agent. On the other hand, the combustion modifications are accomplished by modifying combustion conditions or combustion systems, leading to reduction of NOx formation. Such a representative modification is a lean mix combustion technology. However, such lean combustion has a problem of producing noise, oscillation, and more carbon monoxide and unburnt hydrocarbons, which are responsible for unstable CST combustion during combustion.
These problems can be overcome by using a new technology, that is, the catalytic combustion process. Research focusing on catalytic combustion is now actively underway. Catalytic combustion enables stable complete combustion by a surface reaction of a catalyst even at ultra lean conditions, where stable combustion is hardly accomplished only by using CST combustion. Such catalytic combustion makes it possible to set a CST combustion temperature to a lower temperature at which thermal NOx is not produced. Accordingly, this is an important method to fundamentally solve the problem of nitrogen oxides formation.
Gas turbine combustors using catalytic combustion are designed to have characteristics different from general gas turbine combustors in terms of their combustion reactions. Upon considering realistic application of catalytic combustion to conventional gas turbines, there is considered not only to simple replacement of the combustors, but also design changes for other components composing the gas turbines. Specifically, in addition to combustors of gas turbines, auxiliary components such as external cases, liners with internal holes, fuel injection systems, and ignition-starting systems are also organically affected by such an application of catalytic combustion. Accordingly, given that only combustors of the conventional gas turbines are replaced based on their modular construction, multi-can type combustors are preferentially selected over annular type combustors.
The multi-can type combustors enable convenient control of catalytic combustion for reasons as described above. Also, current trends around the world prefer multi-can type catalytic combustors. In addition, much research has been directed to methods for installing modular catalytic combustors in place of the conventional combustors of the gas turbines. Such replacement of the conventional combustors of the gas turbines with combustors employing catalysts offers the following expected benefits.
First, upon using catalytic combustors, combustion efficiency is higher than conventional combustors of gas turbines at the ultra lean states. The reason is that complete surface combustion takes place on the surface of a catalyst support, unlike conventional processes such as gas flow-dependent combustion. It is expectable that such complete combustion using catalytic combustion provides much solution for satisfying high temperature operation conditions. High temperature operation conditions have been competitively developed by researchers, with the aim of enhancing thermal efficiency.
Another beneficial effect of catalytic combustion is that amounts of NOx, CO and hydrocarbon generated are reduced. Compared to general CST combustion processes, catalytic combustion occurs at a lower temperature, so the amount of NOx generated is reduced. Complete combustion also leads to a reduced amount of hydrocarbon. Such carbon residue reduction may result in decreased abrasion of turbine blades. It is known that a crucial factor determining lifetime of blades in a gas turbine is that carbon residue generated from the combustor damages the blades by collision therewith. Short lifetime of blades thus can be improved by employing catalytic combustion.
Another benefit of catalytic combustion is that hot gas from the combustor has a lower maximum temperature. As a result, cooling requirements accompanying complex structures mounted in conventional gas turbines can be reduced. Actually, upon combustion employing methane for use in operating non-catalytic combustors of gas turbines, a minimum temperature of the combustors is approximately 1000xc2x0 C. This temperature is a minimum temperature capable of being obtained upon the minimum load operation, for all conventional small-sized turbines for industrial use, and most of gas turbines. On the other hand, as for the catalytic combustors, the maximum temperature of the combustor is determined based on a surface combustion temperature, equivalent ratio of gases, pressure and temperature applied in a premix system, and self-ignition temperature in a mixer of the combustor. Such maximum temperature is 1000xc2x0 C. or more. Upon the subsequent CST combustion, the CST combustion temperature can be lowered to a temperature at which little thermal NOx is generated. Thus, it is possible to solve some problems of complex structures of cooling systems in the gas turbines, which might be required due to high temperature in the combustors.
Since the 1970s, in Japan, a research consortium has been organized centering around electric power companies, and research has been dominated by main three groups, that is, a first group including Osaka GAS and Toyo CCI and Kobe Steelworks Co., a second group including Tokyo Electric Power Co. and Toshiba, a third group including Central Research Institute of Electric Power and Kansai Electric Power Co. They developed a catalyst of hexaaluminate whose specific surface area is not significantly reduced even at 1400xc2x0 C., exhibiting resistance to high temperature, and a CST combustor. However, such a ceramic catalyst body has a limitation in terms of thermal shock.
Meanwhile, in the U.S., catalytic combustors for use in gas turbines have been studied since the 1970s. The research has focused on metal honeycomb in which a metal sheet with good thermal shock is coated with a catalyst. Such metal honeycomb is considered to have excellent catalytic effect. For example, Catalytica Energy Inc. was successful in commercializing the first catalytic combustor employing such a catalyst material. The combustor showed performance with ultra low emissions of NOx less than 3 ppm and CO less than 10 ppm. Despite its favorable properties, there are some disadvantages. Since structures of flow baffle boards are different according to the size of combustors, it is inconvenient to design catalyst bodies, and poor fundamental understanding of catalytic surface-chemistry processes limits scaling up of the combustion systems. Moreover, since unstable CST combustion with lift CST combustion is produced in a CST combustion zone, an accurate control of combustion conditions is required.
Therefore, the present invention has been made in view of the above problems associated with conventional combustors, including hot spot, unstable lift CST combustion and flashback, and it is an object of the present invention to provide a hybrid high pressure combustion burner employing catalysts and CST combustion with staged mixing systems, which is technically improved in terms of reduced NOx formation and combustor operations due to CST combustion at lower temperature.
In accordance with the present invention, the above and other objects can be accomplished by the provision of a hybrid high pressure combustion burner with staged mixing systems employing catalysts and CST combustion, comprising: a primary gas mixing portion for homogenous mixing of preheated air with fuel; a catalytic combustion portion for burning the gas mixture prepared in the primary gas mixing portion via surface combustion; a secondary gas mixing portion for mixing of gas burnt via surface combustion in the catalytic combustion portion with unburnt gas which is not subject to surface combustion; and a CST combustion portion for burning the gas mixture prepared in the secondary gas mixing portion.
Preferably, the primary gas mixing portion includes a gas burner or an electric heater for preheating air supplied from a nozzle, a fuel injection nozzle for injecting a fuel, and a primary mixer for homogenous mixing of the preheated air with the fuel.
The catalytic combustion may be achieved in catalytic layers of 4 thin sheets composed of corrugated and straight plates, the flat plate and the corrugated sheet being coated with catalysts, and the 4 sheets being wound to form a snail-shaped structure, as well as a plurality of supporters for supporting the catalytic layers, and an annular part being supported by the supporters.
It is preferable that the secondary gas mixing portion is mounted with a secondary mixer of a swirler. The CST combustion may be achieved at a horn-shaped combustion chamber called the burner tiles.